This invention relates to a thin film electroluminescent device which may be operated at a low voltage to obtain high luminescent efficiency and high brightness.
Electroluminescent devices are generally composed of a transparent substrate, a thin film luminescent layer comprising an electroluminescent phosphor, and two electrodes placed on opposite sides of the luminescent layer. Since such electroluminescent devices are to be employed as display devices, they are required to have a low driving voltage, high brightness and high efficiency. In order to obtain such luminescent properties, a thin film luminescent layer in the device plays an important part and is one of the important factors is production of the luminescent layer.
As a host or matrix compound of the thin film luminescent layers, there have been used compound such as ZnS, or an alkaline earth metal sulfide such as CaS, BaS, SrS and the like. As the luminescent layers of the kind wherein an alkaline earth metal sulfide is used as the host compound, there have been known those listed in Table 1.
TABLE 1 ______________________________________ Host Luminescent compound center element Color ______________________________________ CaS Ce green CaS Eu red SrS Ce blue/green ______________________________________
When producing such luminescent layers by electron beam evaporation, a substrate on which a luminescent layer is formed must be maintained at a temperature of 600.degree. C. or above in order to obtain high brightness since it is impossible to obtain high brightness if thin film luminescent layers are formed on a substrate maintained at a low temperature. However, such a high temperature requires use of the substrate with a high heat resistance and can confer undesired stress on the deposited thin film due to difference in thermal expansion coefficient between the substrate and the thin film. On the other hand, it has been proposed to produce thin film luminescent layers by sputtering. However, brightness sufficient for the practical use can not be obtained with the compositions of the prior art at a low driving voltage and luminescent efficiency is too low to put into practical use.